Using condensed chemicals to precondition lithium niobate and lithium tantalate crystals

ABSTRACT

Methods and apparatus for preconditioning a lithium niobate or lithium tantalate crystal. At least a portion of a surface of the crystal is covered with a condensed material including one or more active chemicals. The crystal is heated in a non-oxidizing environment above an activating temperature at which the active chemicals contribute to reducing the crystal beneath the covered surface portion. The crystal is cooled from above the activating temperature to below a quenching temperature at which the active chemicals become essentially inactive for reducing the crystal.

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/854,536 filed May 25, 2004.

BACKGROUND

The present application relates to preconditioning lithium niobate andlithium tantalate crystals.

Lithium niobate (Li Nb 0₃, “LN”) and lithium tantalate (Li Ta O₃, “LT”)single crystals show a variety of interesting and useful physicalproperties. At room temperatures, LN and LT crystals have aferroelectric order that is a spontaneous electric polarization. The LNand LT crystals also show strong electro-optic coupling,pressure-electricity coupling called piezoelectric effect, andtemperature-electricity coupling called pyroelectric effect.

LN and LT crystals are used in many electronic, optical, andelectro-optical devices, such as surface acoustic wave filters or otherfrequency filters, optical modulators, electro-optical switches, ordetectors using piezoelectric or pyroelectric effects. While aparticular property of an LN or LT crystal may be useful for oneapplication, the same property can have undesired effects in anotherapplication or under certain conditions. For example, a pressure sensorand a heat detector take advantage of the piezoelectric and pyroelectriceffects, respectively. However, if the crystal is subject to excessivemechanical stress or temperature changes during manufacturing oroperation, the piezoelectric and pyroelectric effects can build upelectric charges on the surface of the crystal. The built up chargesdecay slowly, typically in the order of several hours, and can interferewith the operation of the device or even damage the device by triggeringelectric sparks. Such undesired effects can be minimized bypreconditioning the crystals.

In one preconditioning technique, a LN or LT crystal is chemicallyreduced by heating the crystal in a reducing atmosphere. On the surfaceof the crystal, electric charges decay at a higher rate after thereduction than without preconditioning. To accelerate the charge decay,the crystal's reduction can be increased by increasing the temperatureor the duration of the heating during preconditioning. If the decay rateis high enough, surface charges cannot build up to a level that woulddamage the operation of a device including the reduced crystal.

SUMMARY

A lithium niobate or lithium tantalate crystal is preconditioned in anon-oxidizing atmosphere using active chemicals in a condensed materialcovering at least a portion of a surface of the crystal. In thisapplication, “condensed material” and “condensed chemical” refer to amaterial and a chemical in a non-gaseous state, independent of whetherthe material and the chemical have gone through condensation or not.Thus solid or liquid materials are referred to as condensed materialsindependent of how the solid or liquid materials have been prepared.Exemplary condensed materials include crystals, powders, solutions,dispersions and gels.

In general in one aspect, the invention provides methods and apparatusfor preconditioning a lithium niobate or lithium tantalate crystal. Atleast a portion of a surface of the crystal is covered with a condensedmaterial including one or more active chemicals. The crystal is heatedin a non-oxidizing environment above an activating temperature at whichthe active chemicals contribute to reducing the crystal beneath thecovered surface portion. The crystal is cooled from above the activatingtemperature to below a quenching temperature at which the activechemicals become essentially inactive for reducing the crystal.

Particular implementations can include one or more of the followingfeatures. Reducing the crystal can include reducing the crystal by areducing reaction in which one or more of the active chemicalsparticipate. The reducing reaction can include multiple chemicalreactions and at least one active chemical can participate in one of themultiple chemical reactions. The crystal's reduction can be acceleratedby the active chemicals. The crystal can be a crystal wafer. One or moreactive chemicals can be selected from the group consisting of sodiumbicarbonate, potassium carbonate, calcium carbonate, calcium hydride,magnesium carbonate, lithium hydride and lithium carbonate, andcombinations thereof. For example, one or more active chemicals caninclude sodium bicarbonate, calcium hydride, lithium hydride, or lithiumcarbonate. Or one or more active chemicals can include lithium hydrideor lithium carbonate, for example, lithium hydride. The one or moreactive chemicals can include a hydride, such as lithium hydride orcalcium hydride. Or one or more active chemicals can include acarbonate, such as lithium carbonate, potassium carbonate, calciumcarbonate, or magnesium carbonate.

Covering the surface portion with a condensed material can includecovering the surface portion with a condensed material including aninactive component that does not contribute to reducing the crystal atthe activating temperature. Covering the surface portion with acondensed material can include depositing the condensed material bycondensation onto the surface portion. Depositing the condensed materialby condensation onto the surface portion can include depositing thecondensed material during the heating of the crystal in thenon-oxidizing environment. Covering the surface portion with a condensedmaterial can include depositing a thin film of the condensed materialonto the surface portion. Depositing the thin film can include physicalvapor deposition of the condensed material onto the surface portion.Depositing the thin film can include spin coating the surface portionwith the condensed material. Depositing the thin film can include dipcoating the surface portion with the condensed material. Covering thesurface portion with a condensed material can include covering thesurface portion with a powder of the condensed material. Covering thesurface portion with a condensed material can include preparing asolution or dispersion by dissolving or dispersing the active chemicalsin a liquid matrix, respectively, and spinning the solution ordispersion onto the surface portion.

Heating the crystal above an activating temperature can include heatingthe crystal to a temperature that is below a ferroelectric phasetransition temperature of the crystal. Heating the crystal above anactivating temperature can include heating the crystal above about 250Celsius. Heating the crystal in a non-oxidizing environment can includeheating the crystal in a reducing atmosphere or an inert atmosphere.Heating the crystal above an activating temperature can include keepingthe crystal above the activating temperature during a predeterminedactivating time. The activating time can be determined based on theactive chemicals in the condensed material. Cooling the crystal fromabove the activating temperature below a quenching temperature caninclude cooling the crystal from above the activating temperature belowa quenching temperature within a quenching time that is substantiallysmaller than the activating time.

The invention can be implemented to realize one or more of the followingadvantages. By applying condensed chemicals on the surface of a LN or LTcrystal, reduction of the crystal can be substantially acceleratedcompared to a reduction that uses only a reducing atmosphere. Thecrystal can be reduced at a relatively low temperature. In particular,LT crystals can be effectively reduced below the ferroelectrictransition temperature. Therefore, the crystals may not need to be poledduring or after the reduction. The crystal can be reduced usingnon-toxic chemicals that are stable at temperatures used during the L:action. The crystal can be reduced in either a reducing or an inertatmosphere. Entire grown crystals and crystal wafers can be effectivelyreduced. Alternatively, the reduction can be limited to selectedportions of a wafer. The crystal can be reduced using simpletechnologies. For example, the crystal can be covered with a powderincluding the active chemicals, or the active chemicals can be dissolvedor dispersed in a liquid matrix and the solution or dispersion spun ontothe crystal. Alternatively, the chemicals can be deposited on thecrystal by dip coating, spin coating or by physical vapor depositionsuch as sputtering or evaporation.

The details of one or more implementations of the invention are setforth in the accompanying drawings and the description below. Otherfeatures and advantages of the invention will become apparent from thedescription, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flow diagram illustrating a method forpreconditioning LN and LT crystals.

FIGS. 2A-2C are schematic diagrams illustrating exemplary crystalscovered with 5 condensed chemicals.

FIG. 3 is a schematic diagram illustrating a system for preconditioningLN and LT crystals.

FIG. 4 is a schematic diagram illustrating temperature changes duringpreconditioning LN and LT crystals.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

FIG. 1 illustrates a method 100 for preconditioning a LN or LT crystalto increase a decay rate of electric charges on a surface of thecrystal. The LN or LT crystal can be grown, for example, by theCzochralski technique (Current Topics in Material Science, Vol. 1, p.545, edited by E. Kaldis, North Holland Publishing Co., 1978). Themethod 100 can be performed using an entire grown crystal or a pre-cutcrystal, such as a crystal wafer.

Optionally, the LN or LT crystal can be pre-processed before performingthe method 100. For example, the crystal surface can be as-cut, lapped,etched, or polished or the crystal can be poled or un-poled. As-cutsurface refers to a crystal surface that results from a mechanicalshaping or slicing operation. The general surface finishing process mayinclude processing steps such as lapping, etching and mirror polishing.These processing steps can be performed using conventional chemicalmechanical techniques. The crystal can be polished using conventionalchemical mechanical polishing systems. Poling of the crystal can beperformed by heating the crystal above a ferroelectric transitiontemperature (also called Curie temperature Tc, which is about 600Celsius for LT, and about 1140 Celsius for LN crystals) where thecrystal loses ferroelectric order. The crystal is then cooled below thetransition temperature in an electric field that sets an orientation forthe ferroelectric order. Thus the electric field aligns theferroelectric poles of the crystal.

At least a portion of the surface of the crystal is covered with acondensed material including one or more active chemicals (step 110).The condensed material can be in a solid or a liquid phase. In oneimplementation, the condensed material is deposited on the surface ofthe crystal as a thin film. For example, the condensed material can beapplied to the crystal's surface by dip coating, spin coating orphysical vapor deposition. In one implementation, the condensed materialis deposited on a particular portion of the crystal's surface, forexample, after covering the surface of the crystal with a mask patternedto define the particular surface portion. Alternatively, the condensedmaterial can be a powder that covers a surface of the crystal.

For depositing a film on the crystal's surface, the active chemicals canbe dissolved or dispersed in a liquid matrix. In one implementation, theliquid matrix is relatively inert, gas permeable and has a viscositythat can be adjusted to achieve uniform suspension of the activechemicals or to optimize a spin-on process used to cover the crystal'ssurface. For example, the film can be deposited using a gas permeablespin-on glass such as ACCUGLASS®125T-12B available from HoneywellElectronic Materials of Sunnyvale, Calif. This matrix may be uniformlyapplied in a mass production setting using established spin-coating andthermal curing methods.

The condensed material includes active chemicals that contribute toreducing the crystal. The condensed material can also include inertcomponents. For example, the active chemicals can be dissolved in aninert solvent, and the solution can be spun on the surface of thecrystal. Or the active chemicals can be dispersed in a liquid matrix,and the dispersion deposited on the surface of the crystal. Exemplarycrystals covered with condensed active chemicals, and suitable choicesfor the active chemicals, are discussed with reference to FIGS. 2A-2C.

The covered crystal is heated in a non-oxidizing environment above anactivating temperature (step 120). The crystal can be heated using aheating apparatus, such as a furnace discussed below with reference toFIG. 3. In one implementation, the crystal is heated above theactivating temperature in a non-oxidizing atmosphere that is an inert orreducing atmosphere. Alternatively, the crystal can be heated in vacuum.

In one implementation, before being heated in the heating apparatus, thecrystal is already covered with the condensed material that includesactive chemicals for reducing the crystal. Alternatively, the crystalcan be heated above the activating temperature first, and the activechemicals can go through condensation to form the condensed material onthe heated crystal. For example, the active chemicals can be placed inthe heating apparatus near the crystal, and as the temperature rises,the active chemicals can evaporate and the vapor can go throughcondensation to form the condensed material on the crystal's surface.

Above the activating temperature, the active chemicals contribute tochemically reducing the crystal beneath the surface covered with thecondensed material. For example, the active chemicals can participate ina reducing reaction that chemically reduces the crystal. The reducingreaction can include a series of chemical reactions, and the activechemical can participate in one or more of the chemical reactions. Theactive chemicals can participate directly or indirectly in a reactionthat reduces the crystal. For example, the active chemicals canparticipate in a reaction whose reaction product reduces the crystal, orthe reaction product can interact with the reducing atmosphere to reducethe crystal. The active chemicals can also act as catalyzers that,without being chemically changed, accelerate the crystal's reduction.

In general, the term reduction refers to gaining electrons, and the termoxidation refers to removing electrons. For example, removing oxygenfrom a material may reduce one or more components of the material. ForLN or LT crystals, details of the reduction mechanism are still debated.The reduced crystals, however, show a number of characteristic featuresdue to the reduction.

Reduction changes the optical properties of LN or LT crystals. Withoutreduction, the crystal is transparent in visible light. With increasingdegree of reduction, the crystal becomes more and more opaque anddarker. That is, optical transmission decreases in the reduced crystal.Without being bound by any particular theory, the decrease in opticaltransmission might be due to an emerging absorption band centered atabout 2.48 eV (H. Jhans et al, J. Phys. C: Solid State Phys., vol. 19(1986), pp. 3649-3658, incorporated herein by reference).

The changes in optical properties are accompanied by changes in electricproperties of the crystal. With increasing degree of reduction, theelectric conductivity increases in the crystal, and electric chargesinside the crystal can compensate electric charges on a surface of thecrystal. Therefore, surface charges decay faster with increasing degreeof reduction of the crystal. In an appropriately reduced crystal,surface charges decay at a rate that prevents unwanted build up ofelectric charges on the surface of the crystal.

The crystal's degree of reduction depends on a processing temperature towhich the crystal is heated and a duration of the heating. In general,the higher the processing temperature and the longer the duration of theheating, the larger the degree of reduction. In addition, the degree ofreduction is also influenced by properties of the crystal and the activechemicals. Selecting appropriate duration and temperature for theheating is discussed in detail below with reference to FIG. 4 and Table1.

After keeping the crystal above the activating temperature for anactivation period of time, the crystal is cooled below a quenchingtemperature (step 130). Below the quenching temperature, redox reactionsthat are chemical reactions involving reduction or oxidation becomeessentially inactive in normal atmosphere.

FIGS. 2A-2C illustrate exemplary crystals 210, 220, 230 and 240 thathave one or more surfaces covered with a condensed material includingactive chemicals. The crystal 210, 220, 230 or 240 can be an LN or LTcrystal, such entire grown crystals or wafers.

In FIG. 2A, the crystal 210 has two surfaces that are covered by a firstfilm 213 and a second film 216, respectively. As discussed above withreference to FIG. 1, the first 213 and second 216 films can be depositedon the crystal's surface by dip coating, spin coating or physical vapordeposition such as sputtering or evaporation.

In FIG. 2B, the crystals 220 and 230 face each other. Between the twocrystals, a condensed material 225, such as a powder or solid disc, isjuxtaposed so that each of the crystals 220 and 230 has at least onesurface that is covered by the condensed material 225. Alternatively,the condensed material 225 can be deposited on one of the crystals 220and 230, and the other crystal can be positioned to be in contact withthe condensed material 225.

In FIG. 2C, the crystal 240 has a single surface that is covered by acondensed material 245. The condensed material 245 can be deposited onthe crystal's surface by dip coating, spin coating, physical vapordeposition or any other technique. Alternatively, the condensed material245 can include a powder or other solid material.

The condensed materials 213, 216, 225 and 245 include active chemicals.The active chemicals contribute to, for example participate in oraccelerate reactions that reduce the LN or LT crystals. In oneimplementation, the active chemicals include reducing agents that arematerials that typically become oxidized (that is, lose electrons) inchemical reactions. Examples of reducing agents include hydrides, suchas lithium hydride (LiH) or calcium hydride (CaH₂), which effectivelyreduce LT crystals as discussed below with reference to Table 1. Otherhydrides, such as aluminum hydride (AlH₃) or lithium-aluminum hydride 5(LiAlH₄) can also be used for reducing LN or LT crystals.

In alternative implementations, the active chemicals can includecarbonates, such as lithium carbonate (Li2CO₃), potassium carbonate(K₂CO₃), calcium carbonate (CaCO₃), and magnesium carbonate (MgCO₃). Theactive chemicals can also include bicarbonates, such as sodiumbicarbonate (NaHCO₃). The reducing capabilities of different activechemicals are discussed below with reference to Table 1. The condensedmaterial can also include combinations of the various active chemicals.

Appropriate active chemicals for the condensed material can be selectedbased on reducing capabilities or other properties of the chemicals. Forexample, a chemical can be selected based on physical properties thatallow simple processing techniques or provide uniform contact with thesurface of the crystal. Furthermore, advantageous chemicals can includechemicals that are non-toxic or do not decompose at processingtemperatures.

FIG. 3 shows a system 300 for preconditioning LN or LT crystals. Thesystem 300 includes a furnace 310 and a controller 320 that controls thefurnace 310. The furnace 310 can receive multiple LN or LT crystals 330.Each of the crystals 330 has a surface that is covered with a condensedmaterial 340, as discussed with reference to FIG. 2C. The condensedmaterial 340 includes active chemicals that contribute to reducing thecrystals 330.

The furnace 310 includes a heater/cooler element 312, an atmosphereconditioner 314, and one or more sensors 316. The heater/cooler element312 can be used to heat or cool the crystals 330. The atmosphereconditioner 314 can be used to set and maintain properties of theatmosphere inside the furnace 310. In one implementation, the atmosphereconditioner 314 can set and maintain a non-oxidizing atmosphere insidethe furnace 310. For example, the atmosphere conditioner 314 can replacethe atmosphere inside the furnace with a preset gas mixture at apermanent rate. The non-oxidizing atmosphere can be a reducingatmosphere, such as a gas mixture including inert and hydrogen gases, oran inert atmosphere including only inert gases, such as argon ornitrogen. The one or more sensors 316 can detect temperature and/orchemical properties of the atmosphere inside the furnace 310. Thesensors 316 transmit the detected temperature and chemical properties tothe controller 320.

During preconditioning the crystal 330, the controller 320 controls theoperation of the furnace 310. The controller 320 can control theheater/cooler element 312 or the atmosphere conditioner 314, or both.Based on user input or the properties detected by the sensors 316, thecontroller can set an appropriate atmosphere or schedule for the heatingand cooling during the reduction of the crystals 330. In alternativeimplementations, the furnace can be controlled manually, without acontroller.

FIG. 4 illustrates an exemplary heating/cooling recipe graph 400 forpreconditioning a crystal using the system 300 (FIG. 3). The crystal iscovered with a condensed material that includes one or more activechemicals. The graph 400 illustrates the temperature of the crystal as afunction of time. The graph 400 specifies a heating/cooling recipe thatcan be used during the heating and cooling steps of the method 100 (FIG.1).

The recipe graph 400 defines a heating-up portion 410, an activatedportion 420, and a cooling portion 430. During the heating up portion410, the temperature of the crystal rises from below to above theactivating temperature (“T activating”) at a substantially constantrate. Typically, the rate of the temperature rise is between about 5Celsius per minute and about 10 Celsius per minute. Alternatively, thetemperature can rise slower or faster, or can have a varying ratewithout substantially affecting the result of the preconditioning.

In the activated portion 420, the temperature of the crystal remainsabove the activating temperature, where the condensed chemicals becomeactivated and contribute to, for example, participate in chemicallyreducing the crystal under the surface covered with the activechemicals. The activating temperature can depend on the activechemicals. For example, the activating temperature is below 250 Celsiusfor active chemicals including lithium hydride and about 400 Celsius foractive chemicals including calcium hydride.

During most of the activated portion 420, the temperature of the crystalremains at a processing temperature (“T processing”). The processingtemperature can be chosen to be below a ferroelectric transitiontemperature (“T curie”) of the crystal. For example, if the crystal ispoled, the processing temperature can be sufficiently below theferroelectric transition temperature so that the crystal does notrequire a new poling procedure due to the reduction. In particular, anLT crystal has a transition temperature about 600 Celsius. A poled LTcrystal can be effectively reduced using chemicals, such as lithiumhydride or calcium hydride, for which an activating temperature is wellbelow 600 Celsius. Alternatively, the processing temperature can beabove the transition temperature, and the crystal can be poled during orafter the reduction.

In the activated portion 420, temperature is kept above the activatingtemperature for an activation time (“t_a”). Typically, the activationtime is in the order of a few hours. For a given processing temperature,the activation time can be chosen to achieve a desired degree ofreduction of the crystal. The degree of reduction can be monitored bymeasuring an optical transmission or an electric conductivity of thereduced crystal. Alternatively, an optimal activation time can beestimated by comparing degrees of reduction for different activationtimes. Or the activation time can specify a fixed duration, such as fivehours, and an optimal processing temperature can be chosen to achievethe desired degree of reduction. The optimal activation time orprocessing temperature also depends on the active chemicals covering thecrystal.

In the cooling portion 430, the temperature is decreased from above theactivating temperature to below a quenching temperature (“T_quench”).For LN and LT crystals, the quenching temperature is about 100 Celsius.In one implementation, the cooling time (“t_q”) is much shorter than theactivation time. Alternatively, the cooling time can be comparable to orlonger than the activation time. Below the quenching temperature, thecrystal is not reduced or oxidized in normal atmosphere, which is anoxidizing atmosphere. Thus the crystal can be safely removed from thefurnace without altering the effect of the reduction. Alternatively, thenormal atmosphere can be introduced in the furnace before the crystal iscooled below the quenching temperature. Although some reoxidation mayhappen in the normal atmosphere, the crystal can still remain reduced ifit is quickly cooled below the quenching temperature.

Table 1 shows results of a series of experiments that were performed toreduce LT crystals using condensed active chemicals. In theseexperiments, LT wafers were covered with a film including the activechemicals. The film was deposited on the wafers using a dispersion ofthe corresponding active chemical. The coated wafers were heated to aprocessing temperature between 250 Celsius and 580 Celsius in anatmosphere including included 3.5% H₂ and 96.5% N₂. After being heatedto the corresponding process time between about 5 and 6 hour, the LTwafers were cooled to room temperature and their optical transmissionand resistivity were measured.

Without reduction, the LT wafers are transparent, and they becomeprogressively darker with increasing degree of reduction. Thus ingeneral, the less the optical transmission, the more the LT wafer isreduced. However, the optical transmission is also influenced by spatialinhomogeneities of the reduction across the LT wafer. Suchinhomogeneities have less effect for small volumes, and become morepronounced when a larger volume such as the whole wafer thickness isconsidered after a reduction using condensed chemicals on a surface ofthe wafer. Typically, the wafer is most reduced near the surface coveredwith the condensed chemicals, and less reduced in the bulk away from thesurface. Thus a shallow reduction may generate a dark wafer with smalloptical transmission without reducing the bulk of the wafer. The actualreduction profile depends on the details of the reduction process. Forexample, a shallow reduction may be a result of a low processingtemperature. In table 1, the optical transmissions are shown inpercentage relative to optical transmission of the unreduced crystal,which has a 100% transmission in these units.

The resistivities were measured across the wafers at room temperature ina dark room. Although in a different way than for the opticaltransmissions, the resistivities are also influenced by the spatialinhomogeneities of the reduction across the wafer. While the opticaltransmission is typically dominated by the most reduced portions (nearthe surface) of the wafer, the resistivities are typically dominated bythe least reduced portions (in the bulk). Due to differences betweenreduction profiles across the wafers, the optical transmission and theresistivity not necessarily indicate the same amount of reduction for aparticular sample when compared to those of other samples in Table 1.

Furthermore, the resistivity of an LT crystal is highly frequencydependent, and Table 1 specifies the resistivity values for a frequencyof 1 mHz (milliHertz). Such frequency values are typically relevant tosurface charging effects that happen during a few hours. For the reducedsamples, a decrease in undesired charging effects were observed if theresistivity values were below about 5×1014 Ohm cm, and the chargingeffects were substantially decreased if the resistivity values werebelow about 3×1014 Ohm cm, such as below about 30 1×1014 Ohm cm. Thepreconditioned wafers were substantially free of undesired chargingeffects if the resistivity values were below about 0.5×1014 Ohm cm.

As illustrated by Table 1, both the optical transmissions and theresistivities indicate successful reduction of the LT wafers. The LTwafers' optical transmissions and resistivities decreased compared tothe unreduced wafer for each of the listed active chemicals. Therefore,each of these active chemicals can contribute to chemically reducing theLT wafer. For some of the active chemicals, LT wafers were effectivelyreduced at processing temperatures that are well below the ferroelectrictransition temperature of LT, which is about 600 Celsius. For example,the LT wafer was effectively reduced at about 250 Celsius using LiH, orabove 400 Celsius using CaH2. TABLE 1 Resistivity Processing ProcessingOptical at 1 Active temperature time transmission mHz (10¹⁴ chemical(Celsius) (hours) (%) Ohm cm) Li₂CO₃ 580 5 13.5 0.019 Li₂CO₃ 560 6 67.00.29 NaHCO₃ 580 5 15 0.0073 K₂CO₃ 580 5 45 0.025 CaCO₃ 580 5 99 2.04MgCO₃ 580 5 93 1.12 CaH₂ 580 5 24 0.154 CaH₂ 400 5 97 3.5 LiH 250 5 301.15 Not treated N/A N/A 100 11.0

An exemplary experiment for Li₂CO₃ is described below in detail. A filmincluding Li2CO3 was deposited on the LT wafers using a dispersion(slurry). To prepare the dispersion, lithium carbonate powder of 99.99%purity was sifted through a 325-mesh size sieve. The dispersion wasprepared by adding 1.0 gram of the lithium carbonate powder to each 5 mlof liquid matrix. The liquid matrix was a solution of sodium n-dodecylsulfate (SDS) in water prepared by using a ratio of 1 gram of SDS for 10ml of water. The addition of SDS increases the viscosity of thedispersion, slows down undesired settling of the suspended lithiumcarbonate, and improves wetting of the wafer surface.

The LT wafers had been cut, edge-rounded, lapped and etched in HFsolution for 100 minutes at 28 C before depositing the film. The LTwafers were positioned on a chuck of a spin-coating machine, such ascommonly used for semiconductor processing. About 4 ml of the slurry wasdispensed at a spin rate of 400 rpm. The spin rate was then ramped up toa speed of 2000 rpm to evenly distribute the slurry. With thisprocedure, the wafer surface dried within a convenient time frame at aroom or slightly elevated temperature. Thus the wafers could be furtherprocessed without a danger of re-distribution of the chemicals on thesurface. The coated wafers were loaded into a quartz processing boatthat is commonly used in semiconductor processing, and the quartz boatwas loaded into a horizontal tube reduction furnace. Although theloading was done manually at room temperature in the furnace, theloading process can easily be automated and the furnace can be preheatedto a processing temperature.

After loading the wafers, a process gas was introduced into the furnaceat a flow rate of 0.751/min. The process gas included 3.5% H2 and 96.5%N₂. The furnace was heated to 560° C. at a rate of 5 K/min, and held atthis temperature for 6 hours. Next, the furnace was shut off to let thefurnace cool. Within 6 to 8 hours after switching off the power, thefurnace core temperature fell below 80° C. Then, the process gas wasswitched off, the furnace was opened, and the wafers were unloaded. Theprocessed wafers showed a grey color and had increased bulkconductivity. The wafers were rinsed, lightly lapped and polished. Suchpreconditioned wafers were successfully used to produce SAW filters,without any process induced charging effects.

Without being bound by any particular theory, the present inventorssuggest that the active chemical (Li₂CO₃ in the above example) mediatesa reaction between the LT substrate and the gaseous hydrogen in theatmosphere. According to this suggestion, the reduction includes areaction between ambient hydrogen and the carbonate salt, and providesan in-situ generation of an active hydride intermediate (LiH):Li₂CO₃ (condensed)+2H₂ (gas)−H₂O (gas)+CO₂ (gas)+2LiH

The in-situ generated hydride then reduces the LT substrate in asubsequent reaction. Alternative or additional chemical reactions canalso contribute to reducing the LT substrate.

Alternative active chemicals include oxalate salts such as lithium,sodium, potassium, magnesium, and calcium. An example is lithium oxalate(Li₂C₂O₄). The advantage over e.g. lithium carbonate is that significantamounts (up to ˜80 g/l) will dissolve in a water-based matrix. Thisallows uniform application to the wafer surface because there is nosettling of suspended particles, and the active chemical is uniformlydistributed without the risk of conglomerates that may lead to excessivereduction at the point of contact. Furthermore, the concentration caneasily be controlled to give the desired degree of reduction. The activechemical is dissolved in a solution of sodium n-dodecyl sulfate (SDS)that aids in wetting the wafer surface on application.

To prepare the solution, SDS was first dissolved in de-ionized water ata concentration of 100 g/l. The lithium oxalate was then added at aconcentration of 40 g/l of the SDS solution. After dissolution of theoxalate, the solution was filtered with a 6 μm particle paper filter toremove any particles. The solution can be kept for at least 2 weekswithout any noticeable degradation.

The wafers were lapped, etched for stress relief in 48 mol % HF, andthen spin coated with the solution. About 4 ml of solution was dispensedto cover a 100 mm diameter LT wafer while it was spinning at 400 rpm.The spin rate was then accelerated to 2000 rpm in order to get a uniformcoating. The spin-coating lid was left open to aid the drying; thisallowed the wafers to easily be handled after processing. Thespin-coating was generally applied to the wafer surface opposite thesurface that eventually would be polished.

The wafers were transferred to a quartz processing boat with 2.5 mm slotspacing. A typical run had about 100 wafers, but it is expected thatthis number can be increased when using a furnace with a longer zone ofconstant temperature.

The furnace was ramped at 5 K/min to a temperature of 560 C where it washeld for 12 hours before shutting off the heating power. After thewafers had cooled down, the gas was switched off, the wafers wereunloaded, rinsed, and briefly etched in HF again. A second processing ofthe same wafers with identical parameters (spin-coating, processing at560 C for 12 hours) increased the degree of reduction and improved thereduction uniformity across the wafer area as measured by opticalabsorption measurements.

The resistivity of the processed wafers (two runs) was measured at 1 mHzand was around 8×10¹² Ωcm. The optical transmission through the 0.46 mmthick wafers was about 40% of the transmission through an untreatedwafer of same thickness. When putting a treated wafer on a hotplate andchanging the setpoint of the hotplate from room temperature to 70 C, nocharge build-up was observed during heat-up, demonstrating theusefulness of this material.

The invention has been described in terms of particular embodiments.Other embodiments are within the scope of the following claims. Forexample, the steps of the invention can be performed in a differentorder and still achieve desirable results.

1-29. (canceled)
 30. A method for preconditioning a crystal, the methodcomprising: selecting a crystal from the group consisting of lithiumniobate and lithium tantalate crystals; covering at least a portion of asurface of the crystal with a condensed material including one or moreactive chemicals selected from the group consisting of the oxalates oflithium, sodium, potassium, magnesium, and calcium; heating the crystalin a non-oxidizing environment above an activating temperature at whichthe active chemicals contribute to reducing the crystal beneath thecovered surface portion; and cooling the crystal from above theactivating temperature to below a quenching temperature at which theactive chemicals become inactive for reducing the crystal.
 31. Themethod of claim 30, wherein the active chemicals include lithiumoxalate.
 32. The method of claim 30, wherein: covering the surfaceportion with a condensed material includes covering the surface portionwith a condensed material including an inactive component that does notcontribute to reducing the crystal at the activating temperature. 33.The method of claim 30, wherein: covering the surface portion with acondensed material includes depositing a thin film of the condensedmaterial onto the surface portion.
 34. The method of claim 33, wherein:covering the surface portion with a condensed material includesdepositing the condensed material by condensation onto the surfaceportion.
 35. The method of claim 34, wherein: depositing the condensedmaterial by condensation onto the surface portion includes depositingthe condensed material during the heating of the crystal in thenon-oxidizing environment.
 36. The method of claim 34, wherein:depositing the thin film includes physical vapor deposition of thecondensed material onto the surface portion.
 37. The method of claim 33,wherein: depositing the thin film includes spin coating the surfaceportion with the condensed material.
 38. The method of claim 33,wherein: depositing the thin film includes dip coating the surfaceportion with the condensed material.
 39. The method of claim 30,wherein: covering the surface portion with a condensed material includescovering the surface portion with a powder of the condensed material.40. The method of claim 30, wherein: covering the surface portion with acondensed material includes preparing a solution or dispersion bydissolving or dispersing the active chemicals in a liquid matrix,respectively, and spinning the solution or dispersion onto the surfaceportion.
 41. The method of claim 30, wherein: heating the crystal abovean activating temperature includes heating the crystal to a temperaturethat is below a ferroelectric phase transition temperature of thecrystal.
 42. The method of claim 30, wherein: heating the crystal abovean activating temperature includes heating the crystal above about 250Celsius.
 43. The method of claim 30, wherein: heating the crystal in anon-oxidizing environment includes heating the crystal in a reducingatmosphere.
 44. The method of claim 30, wherein: heating the crystal ina non-oxidizing environment includes heating the crystal in an inertatmosphere.
 45. The method of claim 30, wherein: heating the crystalabove an activating temperature includes keeping the crystal above theactivating temperature for a predetermined activating time.
 46. Themethod of claim 45, further comprising: determining the activating timebased on the active chemicals in the condensed material.
 47. The methodof claim 45, wherein: cooling the crystal from above the activatingtemperature below a quenching temperature includes cooling the crystalfrom above the activating temperature below a quenching temperaturewithin a quenching time that is substantially smaller than theactivating time.
 48. The method of claim 30, wherein the crystal is acrystal wafer.
 49. A method for preconditioning a crystal, the methodcomprising: selecting a crystal from the group consisting of lithiumniobate and lithium tantalate crystals: covering at least a portion of asurface of the crystal with a condensed material including one or moreactive chemicals selected from the group consisting of oxalate salts;heating the crystal in a non-oxidizing environment above an activatingtemperature at which the active chemicals contribute to reducing thecrystal beneath the covered surface portion; and cooling the crystalfrom above the activating temperature to below a quenching temperatureat which the active chemicals become inactive for reducing the crystal.50. The method of claim 30, wherein the crystal is lithum tantalate. 51.The method of claim 31, wherein the crystal is lithum tantalate.
 52. Themethod of claim 32, wherein the crystal is lithum tantalate.
 53. Themethod of claim 33, wherein the crystal is lithum tantalate.
 54. Themethod of claim 40, wherein the crystal is lithum tantalate.
 55. Themethod of claim 49, wherein the crystal is lithum tantalate.